/Engine/Plugins/Online/OnlineSubsystemUtils/Source/OnlineSubsystemUtils/Public/PartyBeaconState.h
Obtain player validation string from party reservation entry
true if reservation exists for player
virtual bool GetPlayerValidation ( const [FUniqueNetId](API\Runtime\CoreOnline\Online\FUniqueNetId) & PlayerId, [FString](API\Runtime\Core\Containers\FString) & OutValidation ) const